That looks like the serial connection parameters are wrong in some way, at least based on the LED behaviour. The correct behaviour should be synchronized blinking of the TX and RX leds on the extension, with the default reading frequency once per second (this can be increased via the "scan_frequency" option in the configuration file).

Have you set any of the dip switches on the extensions, and if so, could you take a photo of them? If not then it might be a good idea to hard set 19200 8N1 ('N' parity, 19.2 speed) as well as the address (1 for the xS30, different for the other two extensions [note that in general the addresses do have to be different for each device]).

The dip switches are only read on power-cycling or soft reset, in order to prevent the communication from breaking accidentally during operation, and therefore the extensions need to be power-cycled (or soft reset via EVOK once connection is established).

The error you get back from EVOK indicates that the communication isn't established as well, so I would look at the UART communication layer first.

Edit: The default config is 19200 8N1 address 15, but due to address conflicts this will only work with a single device on the bus.

That is slightly odd, if it causes issues you can try the latest version which should be better about it. Anyway glad to hear you got it working.

Using other/speeds parity than 19.2/8.6 KBd is certainly possible, but it's somewhat difficult to set up. Higher speeds can also be less reliable.

The full range of supported bitrates is 115.6 KBd to 2.4KBd, which is set to be expanded lower, all the way down to 0.1365 (a slightly odd but TERMIOS-standardised speed) KBd, in future firmware which is currently being tested.